The EN 954 standard specifically concerns the requirements for safety-dependent parts of machine controllers. It is part of the harmonised standards of the EU machinery directive. The IEC 61508 standard, on the other hand, regulates requirements for functional safety of electrical, electronic and programmable electronic safety-related systems and is applicable in mechanical engineering as well as in process technology.

The goal of safety technology for machinery and facilities is to prevent industrial accidents. These accidents can be caused by machine failure, faulty operation, or as a result of bypassing safety mechanisms. Appropriate safety systems and observance of the relevant safety standards and guidelines help ensure the worst doesn't happen. The safety systems that are used must be monitored constantly, which is partly a result of the more extensive functionality and increased safety requirements. Safety switching devices perform this task.

So that safety technology achieves its protective purpose to the same extent in all European countries, there is a single and obligatory safety standard: the 98/37/EC machinery directive.

Product or machine manufacturers themselves declare conformity to EN 'harmonised standards.' This declaration in each case applies for all operational machines that are brought into circulation within the European Single Market. Products bearing the CE mark have fulfilled the essential safety requirements of all EU directives concerning that product. In addition, such conformity assessment procedures as are optionally prescribed in the directives have been carried out.

The international IEC 61508 standard 'Functional safety of electrical/ electronic/ programmable electronic safety-related systems' (E/E/PES), although not a harmonised standard of the EU machinery directive, is most definitely an important standard in mechanical engineering. From conceptualisation through to decommissioning, this standard describes all aspects that must be observed to ensure functional safety.

The structure conforms with that of the EN 954-1 standard, but the international standard is much more detailed. To define an acceptable residual risk, both standards contain corresponding hazard and risk analyses. In the EN 954-1 standard, this results in the division into controller and safety categories. In the IEC 61508 standard, the equivalent term is 'Safety Integrated Level' or SIL.

For ascertaining the SIL and safety categories, both standards use virtually identical parameters. The IEC 61508 standard additionally contains a probability consideration. There is a further difference in the safety chain. Only the EN 954 standard allows a separate definition of the individual parts without defining an overall category. By contrast, the IEC 61508 standard also describes a complete safety function.

The international standard IEC 61508 combines individual national regulations specifically for process technology into an internationally recognised standard. This concerns electrical and electronic applications in chemical and process technology as well as those in mechanical engineering.

In everyday operation, the relevant standards must be implemented with support for safety equipment. For this, safety cut-out components such as emergency stop buttons and safety limit switches are used. Although they meet the safety requirements, it is not possible simply to assume that the signal evaluation and wiring for these conventionally wired switching elements work without faults.

Hence, for one-fault safety, as well as the special properties of the components used, those of the wiring also need to undergo an assessment.

Safety relay combinations are implemented for reliable signal evaluation and monitoring. These modules perform cyclical monitoring of the circuitry, enabling them for example to recognise a short-circuit. Then they prevent re-starting and act as a starting lockout after the power is switched back on. In addition, the edge of the V/I characteristic curve can be evaluated for the start and reset button respectively. This makes it possible to identify a welded button, thus preventing unwanted restarting.

The safety relays serve as part of the safety chain for the electrical and hence safe isolation of the field application from the controller. In addition, the safety relays can also carry out voltage and power adjustment.

However, conventional relay modules generally have a high startup current that controllers often interpret as a short circuit. Phoenix Contact offers safety relays whose switching behavior is specially optimised for process technology. The safety relays have two safely isolated redundant normally open (NO) and one normally closed (NC) contact that can switch a voltage up to 250 V AC with a current of 6 A.

–Thomas Hüttemeier, Certified Engineer, Interface Product Marketing Phoenix Contact GmbH & Co. KG, Blomberg, Germany